Purpose:
Following the onset of central vision loss, most patients develop a retinal location outside the central scotoma, the preferred retinal locus (PRL), as the new reference for visual tasks. Little is known as to how the PRL locations are selected. In this study, we tested the hypothesis that the selection of the location for a PRL is based on optimizing visual acuity, which predicts that acuity is the best at the PRL, compared with other retinal locations.

Methods:
Using a Rodenstock scanning laser ophthalmoscope (SLO) as a gaze-contingent display, we first mapped out the absolute scotoma for four observers with long-standing central vision loss (age: 56-84, logMAR acuity: 0.52-1.04). We then measured acuity using a 4-orientation Tumbling-E task presented using the SLO at the PRL and at multiple locations around the scotoma. These locations were positioned along 12 meridians (2-3 positions per meridian) originating from the anatomical fovea (30° apart), straddling 1-5° from the edge of the scotoma. The order of testing these 24-36 locations was random. A staircase procedure was used to track the letter size threshold that corresponded to 71% correct. The thresholds averaged across three staircases represented the acuity at each testing location.

Results:
Across observers, the acuity at the PRL was never the best among all testing locations. Instead, acuities were better (up to 47%) at 10-46% of the testing locations than at the PRL. These locations with better acuities did not cluster around the PRL, and did not necessarily lie at the same distance from the fovea or the edge of the scotoma as the PRL. Except for the only observer who had foveal sparing, acuities did not correlate with the eccentricities of the testing locations. However, when acuities at different eccentricities along the same meridian were averaged, the meridian with the best averaged acuity was consistent with the meridian on which the PRL was located.

Conclusions:
The worse acuity found at the PRL than at other locations around the scotoma implies that the selection of the PRL location was unlikely to be based on optimizing acuity. Although our measurements were made years after the development of the PRL, perceptual learning suggests that with continuous usage, acuity at the PRL should only get better and not worse, further affirming that the initial selection of the PRL location was not based on optimizing acuity at a specific location.